With rapid advance of manufacturing technology, there are many methods being developed for molding plastics into required products, such as injection molding, blow molding, hot embossing molding, compression molding, draw molding, and so on. Among which, injection molding is the most common method of plastic part manufacturing which is used to create a large variety of products with different shapes and sizes, ranged from as simple as a cup to a very complex automotive dashboard, and also ranged from as small as a watch gear weighted only 0.01 gram to a very large bathing tub weighted more than 20 kilograms. Most importantly, they can create products with complex geometry that many other processes cannot, since it is advantageous in its ability of making complex plastic parts at high production rates and high tolerances of repeatability with high precision in dimension. Moreover, the most common material used in injection molding includes polythylene (PE), polypropene (PP), polyvinylidene chloride (PVC), polystyrene (PS), poly(acrylonitrile butadiene styrene) (ABS), and so on.
In a plastic injection molding process, a plastic material is fed into a heated barrel, melted, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the mold cavity. Generally, the mold is only heated to a temperature that is lower than the glass transition point of the plastic material which is to be molded therein, so that the melted plastic is able to solidify to the configuration of the mold cavity and thus form a solid layer on the cavity surface as soon as it come into contact with the cavity surface. It is noted that the ratio between the thickness of the solid layer and that of a final product of the injection molding process is increasing with the decreasing of the final product's thickness. However, if the ratio is too larger, i.e. the solid layer is too thick comparing with that of the final product, it will cause difficulty for feeding the melted plastic into the mold and thus plastic injection molding process will have to deal with problems, such as short shot, incomplete molding, and residual stress, etc.
Except for the injection unit, control unit and hydraulic unit, an injection molding device uses a clamping unit for molding the melted plastic into final products. The clamping unit usually is comprises of a pair of matching first clamping seat and second clamping seat, which can also be referred as a front molding board and a rear molding board. As the male die and female die are arranged respectively on the first clamping seat and the second clamping seat, the male die and the female die can be integrated tightly by the clamping of the first and the second clamping seats so that the operation of plastic filling and cooling process for injection molding can be proceeded.
In the aforesaid conventional injection molding process, a thin solid layer will be formed on the cavity wall as soon as the melted plastic being filled into the cavity is in contact with the cooler cavity wall, and the same time that the temperature of the melted plastic in the cavity that is flowing in the neighborhood of the cavity wall will drop. Accordingly, the closer to the center of the cavity, the faster the melted plastic is going to flow as it is almost static near the cavity wall. It is noted that such inconsistent flowing condition for the melted plastic flowing inside the cavity will cause turbulences to be generated in the cavities of the male die and the female die even when the structures of the two are only a slightly more complex, and consequently, there can be ripples and lines being formed on its final products. The ripples and lines along with bubbles caused by the air trapped inside the cavities are going to cause severe defect to the outlook of the final products.
In response to the smaller, thinner and lighter trend for the modern 3C products, a more advanced plastic injection molding process is in demand for satisfying the requirement of producing products configured with microstructures measured in hundreds of micrometers or even tens of micrometers, such as backlight panels, fiber optic connecters, etc., that can not be manufactured by conventional plastic injection molding as it is troubled by the molding conditions of flowability and plastic solidification while being used for manufacturing the aforesaid products configured with microstructures. For overcoming such difficulties, there are many studies relating to how to raise the cavity temperature in a mold rapidly, and some of which even come up with a cooling method along with the rapid heating method so as to reduce the time of an injection molding cycle. The aforesaid rapid heating methods currently available can be categorized into three types according to the heating means, which are steam heating type, electrical resistance heating type and high-frequency heating type. For clarity, several electrical resistance heating devices and high-frequency heating devices will be provided hereinafter for illustration.
There is a mold heating/cooling device being disclosed in TW Pat. No. M317917. Using the aforesaid device, when the first and the second clamping seats are clamped for integrating a first die and a second die of a mold and as soon as a melted plastic is being fed into the cavities of the mold, the heater of the device will be powered to heat up while enabling the heat of the heater to be transferred through the second die to the melted plastic inside the cavity, by that the melted plastic is able to flow at the same speed inside the cavity without causing any turbulence. Therefore, the aforesaid device is able to eliminate the formation of ripples, lines and bubbles on the products of injection molding. However, although the aforesaid device is able to raise the temperature of the mold rapidly, its heating efficiency can be very low since the heat from the heater can be dissipated by the mold and lost before being conducted thereby to the melted plastic. Moreover, since the temperature of a location in the mold that is closer to the heater will be higher than that farther to the heater, the melted plastic at different position in the mold might still flow at different speeds.
In a method and apparatus for heating mold by high-frequency induced current disclosed in TW Pat. No. I279304, there are holes near the heated surface in the mold and the coils can be installed into the holes. The coils surround the heated surface and are conducted with high frequency current. Due to the directional change of the current, the blocks that are surrounded by the coils will be heated by the hysteresis losses and the eddy-current losses. The surface of the mold insert or cavity will be heated rapidly. There are cooling holes set near the heated surface or beside the coil-pipe. The cooling liquid or air can flow in the holes to carry out extra energy and the temperature of the mold will be decreased. The position of the cooling holes, the flow speed and temperature deviation of the liquid and air will influence the temperature of the mold. However, to use the magnetic field heating properly it is importance to control the distance between any two neighboring coils. It is noted that when the neighboring coils are positioned too close to each other for enabling the coils to generate heat in a uniform manner, the interaction between the magnetic fields caused by the currents flowing in the neighboring coils will cause the heating efficiency of the heating device to drop. Nevertheless, when the distance between neighboring coils is increased for improving heating efficiency, the melted plastic in the mold may be heated not evenly and thus might be flowing at different speeds
In a method and device to increase distribution uniformity of magnetic force disclosed in TW Pat. No. I 228945, the coil body is coiled in such a way that it appears to have undulating and even distributed multi-layer structure. As there is a pluralities of neighboring coils formed on different layers of the coil body, magnetism goes through any two neighboring coils will not repel or counteract each other because the neighboring coils are not on the same plane. Thus the aforesaid device can improve the uniformity of its high cycle wave magnetic field distribution. However, since coil body is coiled as an undulating and even distributed multi-layer structure provided for different coils to be disposed thereof at different layers, it will require the aforesaid device to be designed with a larger space for accommodating the coil body so that the overall size of the device will be increased and thus the heating distance is increased in the consequence of unable to shorten the heating cycle.
Therefore, the focal point of the present invention is how to heat up a mold or inserts evenly in relatively shorter period of time and cool down the same thereafter as well.